1. Field of Invention
The present invention relates to a method for the continuous casting of a hollow metallic ingot and an apparatus therefor. More particularly, the present invention relates to a method and
apparatus for the continuous casting of a hollow ingot having a smooth cast skin on the inner peripheral surface and a small inverse segregation-layer.
The present invention also relates to a casting-start method in the continuous casting of a hollow ingot, in which the following troubles, which occur at the beginning of casting of a hollow ingot, are prevented: the core is encased in the cast metal; solidified thereon; and, the molten metal flows out through an inner peripheral part of the hollow ingot.
2. Description of Related Arts
A metallic tube and a long hollow material are used as the final products in their forms. Besides, a metallic tube and long hollow material are indispensable as blank materials of various annular or tubular members, such as wheel rims of vehicles, cylinders of compressors and the like. This blank material is produced by a die-extrusion method of a columnar metallic ingot, a continuous casting method, or a centrifugal casting method. The continuous casting method provides, at a low cost, a hollow ingot, having homogenous and fine casting structure free of a texture generating a directional property. The continuous casting method is therefore appropriate for producing the blank material which is subjected to plastic working, such as or ring-rolling or forging.
The continuous casting of a hollow ingot is generally carried out as follows. A core or a mandrel (hereinafter collectively referred to as the core) is concentrically held within a forcedly cooled, tubular mold to form an annular clearance between the mold and core, and the molten metal is continuously poured into the annular clearance. A solidified shell is formed around the hollow metallic metal in the mold. The solidification then proceeds toward the interior of the ingot, while it issues outside the mold and is directly sprayed by cooling water. The thus formed hollow ingot is withdrawn outside the mold at a controlled casting speed. The above described continuous casting method is generally embodied as: the so-called float casting method, in which a refractory floating member is disposed on the level of molten metal in a tubular mold so as to control the pouring quantity of molten metal; and, the so-called hot-top method, in which a relatively deep, refractory reservoir of molten metal is located integrally on the top of a forcedly cooled mold, and the level of molten metal in the reservoir is adjusted to the same level as that in the though for feeding molten metal to the reservoir. In a casting method without the aid of a mold, the molten metal is held in the columnar form by means of magnetic force and the columnar body is directly subjected to the water cooling to solidify the same. This method is implemented in a limited field of continuous casting. In the commercial casting, a multi-strand casting with a number of molds and the like arranged in parallel is carried out with regard to each of the above mentioned methods.
The above described continuous casting methods for a hollow ingot can be distinguished from one another from the view point of the following kinds of core: (A) refractory, non-cooled core; (B) forcedly cooled core; and (C) a core, in which an electromagnetic force is applied for shaping. Methods belonging to (A), above, are the floating method (1) and the float casting method (2). In the floating method (1), a gas-permeable core made of plaster is preliminarily shaped in an elongated form, molten metallic ingot is poured between the core and mold to form a hollow metal and to bond the metal with the core, and a solidified, long ingot is withdrawn outside the mold together with the bonded core, and subsequently the hollow metallic ingot and core are disassembled from one another (c.f. Japanese Examined Patent Publication No. 35-1106). In the float-casting method (2), a refractory core is made of material which is difficult to be wetted with molten metal and is maintained within a mold at a predetermined level (c.f. Japanese Examined Patent Publication No. 55-42655). Methods belonging to (B), above, are the following (3), (4) and (5). In the method (3) disclosed in Japanese Unexamined Patent Publication No. 57-127584, vibration is imparted to a core by means of an electromagnetic vibrator and the like during casting. In the method (4) disclosed in Japanese Unexamined Patent Publication No. 56-141944, a rotary core is used and is provided on the outer peripheral surface with a longitudinal slit for feeding lubricating oil on this surface. In the method (5) disclosed in Japanese Unexamined Patent Publication No. 57-181759, a refractory core used is provided with cooling conduits embedded therein. Methods belonging to the (C), above, are the following (6) disclosed in U.S. Pat. No. 4126175, in which an inductor disposed in a water-cooled mold generates electromagnetic force for forming the inner peripheral surface of molten metal and the ingot is brought into contact with neither the mold nor core but is directly water-cooled.
The qualities required for a hollow ingot, particularly one subjected to plastic working, such as forging, ring rolling, swaging and the like, are smooth cast skin of the inner peripheral surface and fine and homogenous structure with few inverse segregations. The additional qualities required for a hollow billet are roundness of the hollow part and uniformity in thickness of the round wall part of the billet. When these qualities are not fulfilled, a hollow billet needs to be subjected to machining for removing the inner peripheral surface layer, in large quantity. This necessitates and increase during casting in the thickness of the hollow ingot or the like by an amount corresponding to the destined machining depth. The machining cost is additionally required. A great amount of machined chips are lost during remelting thereof. Consequently, the cost increase incurred due to the above machining is serious. Furthermore, when the hollow part of a long ingot having a small-diameter is machined, the operation is so difficult that productivity is reduced.
The above described, conventional methods for continuously casting a hollow ingot have merits and demerits. It is difficult by means of these methods to industrially stably produce hollow ingots thoroughly fulfilling the above described properties. In methods (1) and (2), homogeneous structure is not obtained.
In addition, since the core of method (1) is consumable, the cost is disadvantageously increased. Since it is difficult to prevent the leakage of molten metal through the solidified shell at the side of the core, a stable operation is difficult in method (2). Method (3) is effective for reducing the engulfment of superficial oxide film. Leakage of molten metal through thin solidified shell is however likely to occur and, therefore, formation of smooth cast skin is difficult. In method (4), stable rotation of a core is difficult, since molten metal shrinks on the water-cooled core and exerts fastening force impeding the rotation of the core during solidification thereof. The rotary movement and lubrication are therefore not effective for forming smooth cast skin on the inner peripheral surface. In method (5), cooling conduits, through which air and the like are blown, are embedded in a core to control the temperature of the core. This method involves, as in methods (1) and (2), the drawbacks of leakage of molten metal at the core. Method (6) is effective for lessening the surface defects and inverse segregation of a hollow ingot but necessitates expensive installation expenditure for generating the electromagnetic field. In this method, the distance between multistrand molds are limited and the roundness of an ingot is impaired. Furthermore, since an inductor is assembled in the core, the space required therefore makes it difficult to reduce the size of the core. This method is therefore not applied for the production of ingots having a small-diameter hollow part.
When the continuous casting operation is to be started, a tubular water-cooled mold is closed at its withdrawal end by a movable bottom block which is capable of displacing in the casting direction. Molten metal is then continuously poured into the mold cavity formed between the tubular mold and core. The poured molten metal successively solidifies in the mold cavity and then forms a bonding part with the movable bottom block which has been placed at the beginning to close the withdrawal end of the mold cavity. Upon arrival at this condition, the movable bottom block is caused to displace so as to withdraw the hollow ingot. During withdrawal, the cooling water is injected onto the inner and outer peripheral surfaces of the hollow ingot to cool it. The spontaneous cooling of the hollow ingot without injection of cooling water may be occasionally carried out. Upon initiation of the above outlined start of casting, the tapping temperature of the molten metal, cooling water-flow rate in the mold and the like are monitored to estimate the solidification timing of molten metal on the movable bottom block. During the displacement of the movable bottom block, its speed is controlled in a delicate manner. For performing the sequence of start operations under the present circumstances, the skill of operator is indispensable. Although the casting start is carried out based on experience, such casting parameters as tapping temperature may vary beyond the criterion range. In this case, the molten metal solidifies due to drastic cooling by core and rigidly encases the core. Alternatively, when the cooling by the core is weak, the solidified shell is too thin to hold the molten metal therein. In this case, the molten metal may flow out of the solidified shell on the core. The continuation of the casting operation becomes difficult due to such trouble.
Incidentally, it is important for stabilizing the casting start and for providing smooth cast skin to provide the core with such a draft that diameter is great at the top part (inlet of metal flow) and is small at the bottom part. When metal solidifies and shrinks during the continuous withdrawal of an ingot, the friction resistance is caused between the outer peripheral surface of a core and the solidified shell or molten metal's surface destined to form the hollow surface. Since the solidification and shrinkage are intensified in the casting direction, the friction resistance is increased at a lower part of core. The draft of core can mitigate the friction resistance. With increase in draft, its effect becomes great but particular casting defects, i.e., lapping pattern or dropping pattern of unsolidified molten metal, become liable to form on the inner peripheral surface of a hollow ingot. When the draft is too small, the friction resistance is increased to a level where cracks are formed on the inner peripheral surface of a hollow ingot. Molten metal may leak through the cracks. The core may then be rigidly encased by the leaked molten metal, which makes the casting operation impossible. The draft of a core is therefore determined, depending upon the respective kinds of alloy and dimension of hollow ingots within an optimum range for attaining criterion qualities of cast skin.
The trouble of rigid core-encasement is most likely to occur in the case of using a forcedly cooled core, because the solidified shell rapidly grows on the forcedly cooled core during the initiation period of casting. Thickness and height of the solidified shell vary locally on the movable bottom block, because the cooling intensity of the molten metal varies depending upon the position in the mold cavity, such as inflow position and its opposite position. The casting parameters, such as the descending timing of the movable bottom block and the like, are therefore set within narrow ranges. It is very difficult to start the casting of a hollow ingot with a thin wall ranging from approximately 8 to 50 mm by means of a mold equipped with a forcedly cooled core. Meanwhile, heat conductivity of the heat-insulative core is dependent upon the material and dimensions of the particular core used. In the case of a graphite core, which is generally known core, the heat conductivity is high as compared with the refractory and heat-insulative core. When the graphite core is used at normal temperature, similar troubles as encountered in a forcedly cooled core, are liable to occur. The graphite core is therefore occasionally preheated before using. This preheating is not only very complicated in the mass production of hollow ingots but is extremely difficult to attain always constant range of temperature of cores at the casting start.